1. Field of the Invention
The present invention relates to a thermal printer for forming an image by a thermal head having a plurality of heating elements disposed in line, and more particularly to a resistance value measuring device for measuring a resistance value of a heating element, a method of driving a thermal element in accordance with its resistance value, and a thermal printer.
2. Description of the Background Art
As well known, there are thermal transfer printers and direct thermal printers. In the case of a thermal printer, the back of an ink ribbon or ink film is heated by a thermal head to transfer ink from the ink film to an image receiving sheet. Thermal printers are classified into a thermal die transfer type and a thermal wax transfer type. In the case of a direct thermal printer, a thermosensitive recording medium is heated by a thermal head to directly form an image on the thermosensitive recording medium. These thermal heads have a heating element array having a number of heating elements (resistive elements) disposed in line and a driver for driving each heating element.
As described, for example, in U.S. Pat. No. 4,734,704 (corresponding to Japanese Patent Laid-open Publication No.61-213169), a color direct thermal printer uses a color thermosensitive recording medium having a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, and a yellow thermosensitive coloring layer, laminated in this order on a base. Each thermosensitive coloring layer has its specific coloring heat energy so as to selectively develop its color. The deeper the thermosensitive coloring layer, the higher coloring heat energy is required. After a thermosensitive coloring layer is thermally recorded, the next underlying thermosensitive coloring layer is thermally recorded. In this case, in order not to thermally record again the already recorded layer, this layer is optically fixed by applying a specific electromagnetic wave.
Each heating element of a thermal head supplies a coloring heat energy to a color thermosensitive recording medium in accordance with a characteristic curve of each thermosensitive coloring layer, and forms an ink dot having a desired density on each virtual square record pixel of the color thermosensitive recording medium. The coloring heat energy includes a heat energy immediately before a thermosensitive coloring layer develops color (this heat energy is hereinafter called a bias heat energy) and another heat energy for developing color at a desired density (this heat energy is hereinafter called an image heat energy). The bias heat energy has a constant value determined by the kind of a thermosensitive coloring layer. The image heat energy changes with image data. Image data represents a tonal level of a pixel. The larger the image heat energy, the higher the coloring density of an ink dot. In order to provide a high gradation, the image heat energy is required to be controlled at a fine step.
The bias heat energy is generated by driving a heating element by one bias drive pulse. The image heat energy is generated by driving a heating element by image drive pulses corresponding in number to image data. Generally, the width of a bias drive pulse is several ms to several tens of ms, and the width of an image drive pulse is several .mu.s to several tens of .mu.s. During a bias heating period, a heating element may be continuously driven by one bias drive pulse, or it may be intermittently driven by a plurality of bias drive pulses.
Also in the case of the die transfer type, used for transferring ink at a desired density onto each record pixel of a recording sheet (image receiving sheet), there are at least one bias drive pulse for heating immediately before the start of die transfer and image drive pulses corresponding in number to image data for adjusting a die transfer amount. In the case of a wax transfer type, gradation is represented by changing an area of an ink dot transferred to a record pixel. Used in this case are a bias drive pulse for heating to a temperature at which ink transfer starts and a plurality of image drive pulses each maintaining this temperature. Each time an image receiving sheet is fed by one sub-line, the image drive pulses are supplied to a heating element to transfer ink to each sub-line. A single record pixel is constituted by a predetermined number of sub-lines, and the density of a record pixel corresponds to the number of sub-lines to which ink is transferred.
In order to correctly reflect a fine heating control upon a print result, it is necessary that the resistance values of all heating elements are uniform. However, the resistance values of heating elements have a variation of about 5 to 10%, resulting in undesired phenomena such as a density variation, a color variation, and other variations of record pixels. The resistance value of each heating element is designed to have a value of, for example, 2400 ohms so that a resistance error is 120 to 240 ohms.
In order to eliminate such undesired phenomena, a thermal printer as described, for example, in Japanese Patent Laid-open Publication No.2-248262 has been proposed. With this thermal printer, the resistance values of all of several hundred heating elements are measured and image data is corrected by the measured results. A divider resistor having a known resistance value R is connected between the heating element and a power source. A first switch is connected in parallel with the divider resistor. The first switch is turned on during a print mode and turned off during a resistance value measuring mode. A second switch is connected serially to a noise absorbing capacitor which is connected in parallel with the heating element array. The second switch is turned off during the resistance value measuring mode.
During the resistance value measuring mode, the first and second switches are turned off to enable the divider resistor and disable the noise absorbing capacitor. Under this condition, a transistor serially connected to a heating element is turned on to apply a power source voltage E only to this heating element whose resistance value is to be measured. While the heating element is heated, a voltage V across the heating element array is measured. The resistance value Ra of each heating element is calculated from the following equation. EQU Ra=V/(E-V)!.multidot.R
The heating elements are sequentially powered in the above manner to measure the voltages and obtain the resistance values of all the heating elements. In accordance with the obtained resistance values of the heating elements, image data is corrected to compensate for a heat energy error caused by a resistance value error of each heating element, so that an ink dot can be recorded at a density corresponding to the image data.
This thermal printer requires the switch for disabling the noise absorbing capacitor during the resistance value measuring mode, and in addition because of the disabled capacitor and external noises, a correct resistance measurement becomes difficult. Furthermore, measurement of the voltage E requires, for example, an A/D converter, complicating the circuit.
These problems can be solved by a resistance value measuring device described in U.S. Ser. No. 08/113,807 filed on Aug. 31, 1993, now U.S. Pat. No. 5,469,068. This device uses a noise absorbing capacitor even during the resistance measurement. In this application, two embodiments are disclosed. In the first embodiment, the resistance value of a heating element is obtained from a discharge time of a noise absorbing capacitor discharging current through the heating element. In the second embodiment, the resistance value of a heating element is obtained from a discharge time through the heating element and from a discharge time through a standard resistor connected in parallel with the heating element array.
In the first embodiment, the resistance value calculated by a resistance value calculation equation containing a capacitance term of the noise absorbing capacitor has an error to be caused by an error of the capacitance value. In the second embodiment, the resistance value calculation equation contains a resistance term of the standard resistor and does not contain a capacitance term of the noise absorbing capacitor. As a result, although an error of the capacitance value of the noise absorbing capacitor is not present, an error of the resistance value of the standard resistor influences the calculation result. A capacitor having a high capacitance precision is more expensive than a resistor having a high resistance precision. Therefore, the second embodiment is preferable in that the resistance value measuring device becomes cost effective.
The second embodiment using the standard resistor will be described with reference to FIG. 34, which is herein incorporated for reference. A thermal head 2 has a heating element array 3, a drive IC 4, and a noise absorbing capacitor 5. The heating element array 3 has a number of heating elements 3a to 3n disposed in line. The drive IC 4 is an integrated circuit manufactured by semiconductor integration technology, and has a number of transistors 4a to 4n serially connected to the heating elements 3a to 3n. These transistors 4a to 4n control the conduction of the heating elements 3a to 3n.
In this resistance value measuring device, an external serial circuit of a standard resistor 6 and an additional transistor 7 is connected to a commercially available thermal head 2. The resistance value Rs of the standard resistor is known. Reference numeral 8 represents a switch such as a FET switch, and reference numeral 9 represents a comparator. The other circuits will become apparent when reading the preferred embodiments of this invention, and so the description of the other circuits is omitted.
First, the switch 8 is turned on to connect the noise absorbing capacitor 5 to the power source and charge it to the rated voltage E of the power source. Next, the switch 8 is turned off and the additional transistor 7 is turned on. As a result, the noise absorbing capacitor 5 discharges current via the standard resistor 6. During this discharge, a time Ts required for the rated voltage E to lower to a reference voltage Vref is measured (refer to FIG. 35). This time can be known by measuring a lapse time from when the additional transistor 7 was turned on to when a comparison signal from a comparator 9 changes. Similarly, the discharge times of the heating elements are sequentially measured. For example, assuming that the discharge time of the heating element 3a is Ta (refer to FIG. 35), the resistance value Ra of the heating element 3a can be obtained from the following equation (1) by using a discharge time ratio Ta/Ts and the resistance value Rs of the standard resistor 6. EQU Ra=(Ta/Ts).multidot.Rs (1)
If the saturation voltages of the additional transistor 7 and the transistors 4a to 4n of the drive IC 4 are different, a measurement precision of a resistance value becomes poor. It is difficult to have the same saturation voltage because the additional transistor 7 and the transistors 4a to 4n are manufactured at different processes. The influence of the saturation voltage upon the resistance value measurement will be detailed. The transistors 4a to 4n of the drive IC 4 are manufactured by semiconductor integration technology so that the saturation voltages can be presumed generally uniform although they are different in the strict sense. The saturation voltage of the transistors is represented by Vcen. The saturation voltage Vces of the additional transistor 7 is generally different from Vcen. A difference between Vcen and Vces is the cause of a measurement error.
The relationship between Vces and Vref is given by the following equation (2) in which the time Ts is a time required for the voltage of the noise absorbing capacitor 5 to lower to Vref during the discharge via the standard resistor 6. EQU Vref=(E-Vces) exp (-Ts/CRs)+Vces (2)
Modifying the equation (2), we obtain the equation (3). EQU C=(-Ts/Rs)/ln {(Vref-Vces)/(E-Vces)} (3)
For the discharge via the heating element 3a having an unknown resistance value Ra, the following equation (4) is obtained and the equation (5) is derived from the equation (4). EQU Vref=(E-Vcen) exp (-Ts/CRa)+Vcen (4) EQU Ra=(-Ta/C)/ln {(Vref-Vcen)/(E-Vcen)} (5)
By substituting the equation (5) into the equation (2), the following equation (6) is obtained. ##EQU1## where EQU .alpha.1=ln {(Vref-Vces)/(E-Vces)}!/ln {(Vref-Vcen)/(E-Vcen)}!
.alpha.1 results from a difference of the transistor saturation voltage between Vcen and Vces, and causes a measurement error of the resistance value. The equation (1) of the first embodiment described above assumes .alpha.1 is 1, i.e., Vces=Vcen. However, the saturation voltages Vcen and Vces are generally different and a correct resistance value Ra cannot be obtained.
The above-described first embodiment calculates the resistance value Ra from the discharge time Ta via the heating element 3a without using the standard resistor. EQU Vref=E/exp (-Ta/CRa) (7) EQU R=(-Ta/C).multidot.ln (Vref/E) (8)
where C is the capacitance of the noise absorbing capacitor. If Vref=E/2, then the resistance value is given by: EQU R=Ta/0.693 C
A measurement error of the second embodiment results from a difference between saturation voltages of the transistors, and a measurement error of the first embodiment results from a saturation voltage of the transistor connected to a heating element to be measured. Representing the saturation voltage of the transistor 4a connected to the heating element 3a by V0, the following equation (9) is obtained. EQU Vref=(E-V0) exp (-Ta/CRa)+V0 (9)
Modifying the equation (9), we obtain the following equation (10). EQU Ra=(-Ta/C)/.alpha.2 (10)
where .alpha.2=ln (Vref-V0)/(E-V0)!. PA1 If Vref=E/2, then .alpha.2 is given by: PA1 .alpha.2=ln (Vref-V0)/(2Vref-V0)!
An error .alpha.2 is contained in the resistance value of a heating element calculated only from the time Ta required for the noise absorbing capacitor 5 to lower to the voltage Vref. For example, if E=20 V, Vref=15 V, and V0=0.3 V, the calculated resistance value becomes smaller by about 1.77%.
In addition to Japanese Patent Laid-open Publication No.2-248262 in which image data is corrected in accordance with a measured resistance value, another method as disclosed in Japanese Patent Laid-open Publication No. 2-292060 has been proposed in which density correction drive pulses corresponding in number to a resistance value are inserted between image drive pulses.
However, correcting image data in accordance with a resistance value and changing the number of image drive pulses or the pulse width require a great amount of calculations of all pixels of each frame and a high speed calculation circuit, resulting in a high manufacturing cost. If the calculation results are quantized and converted into the number of image drive pulses, a large quantization error is generated and a quasi contour is formed on a print image, lowering a print quality.
The method of inserting density correction drive pulses between image drive pulses requires a new circuit for generating a density correction drive pulse, resulting in a complicated circuit and a high manufacturing cost. Furthermore, a print time is elongated because density correction pulses are inserted between image drive pulses.